CO2 lasers have been used for decades in industrial processes, wherein the infrared (IR) wavelength and relatively high power of a CO2 laser is advantageous. The lasing medium in a CO2 laser is a gas-discharge in a gas mixture. The gas mixture typically includes 10% to 20% CO2 and is maintained at less than one atmosphere of pressure. The gas mixture is energized to generate the gas-discharge by applying an electric current or a radio-frequency (RF) field between two electrodes. CO2 lasers can deliver output laser-radiation at infrared wavelengths within a range from about 9 micrometers (μm) to about 11 μm. A CO2 laser may be configured as a waveguide-laser or a slab-laser.
In a waveguide CO2 laser, the gas-discharge is established within a long and comparatively narrow waveguide. A laser-resonator is formed around the energized gas mixture by resonator mirrors, located at each end of the waveguide, that direct laser-radiation a longitudinal direction. The narrow waveguide constrains one or more laser-radiation modes in two mutually-orthogonal transverse directions. Laser-radiation is amplified by stimulated emission during multiple passes through the gas-discharge. Such CO2 waveguide-lasers are capable of providing an inherently high-quality output beam, with good power and wavelength stability, but at relatively low average powers. Typically, at an average power less than about 150 Watts (W). Such a waveguide CO2 laser is described in U.S. Pat. Nos. 6,192,061 and 6,788,722, each thereof owned by the assignee of the present invention and the complete disclosure of each is hereby incorporated herein by reference.
In a slab CO2 laser, the gas-discharge is established in a volume between flat wave-guiding surfaces of two closely-spaced electrodes. A laser-resonator is formed around the energized gas mixture by two resonator mirrors. In one transverse direction, the small gap (“discharge-gap”) between the two electrodes defines a waveguide that constrains laser-radiation modes. In the orthogonal transverse direction, the resonator mirrors typically define an unstable laser-resonator. Laser-radiation exits the unstable laser-resonator as an approximately collimated beam, passing through a hole in one of the resonator mirrors or passing by an outside edge of one of the resonator mirrors. Such a slab CO2 laser is described in U.S. Pat. Nos. 6,256,332 and 7,263,116, each thereof owned by the assignee of the present invention and the complete disclosure of each is hereby incorporated herein by reference.
Such slab CO2 lasers are capable of delivering an output beam at an average power of up to about 8 kilowatts (kW). However, the output beam inherently has an elongated cross-section. Additional beam-conditioning optics are required to transform the output beam into a more useful cross-sectional shape. For example, a circular shape.
Slab CO2 lasers are usually operated in a pulsed mode, delivering pulses of laser-radiation having high peak powers. In many industrial laser-processes, higher average power translates to higher throughput. Process efficiency justifies a laser cost that scales with average power. Usually, the cost scaling results from a need to provide complex cooling arrangements to cope with waste heat generated when operating at high average power. Typically, a fluid coolant is forced through channels within one or both of the electrodes. Alternatively, air is forced through an array of metal fins that are in direct thermal contact with one or both of the electrodes. Liquid water is a preferred coolant in many applications because water has a comparatively high heat capacity for efficiently removing substantial waste heat and because fluid-cooling generates less acoustic noise than equivalent forced-air cooling.
In recent years, pulsed slab CO2 lasers have been used increasingly in dental applications, including both hard-tissue and soft-tissue operations, where the relatively long IR wavelength permits cutting and coagulation at the same time. The relatively high cost of industrial slab-lasers can be prohibitive for a dental practice, where other surgical alternatives are available, albeit less convenient for the surgeon and often more painful for the patient.
Dental operations using CO2 lasers usually require a relatively high peak power, but this can be at a relatively low duty-cycle, so a high average power has little utility. There is a need for a CO2 slab-laser that can deliver radiation at a high peak power, but at a relatively low average power, thereby eliminating the need for the complex cooling arrangements that increase cost.